The present invention relates to the control and monitoring of heating cables.
Heating cables, such as XL-Trace.TM. self-regulating heaters manufactured by Raychem Corporation of Menlo Park, Calif. , are specially designed for freeze protection of metal and plastic water pipes in commercial, institutional and residential construction applications.
These heating cables are self-regulating in that two parallel bus wires are separated by a self-regulating conductive core. Current flows between the two bus wires independently at each point along the heating cable. The self-regulating conductive core of the heating cable continuously adjusts its heat output in response to pipe temperature, providing a freeze protection system that gives the user the correct amount of heat at each point along the pipe. It is apparent that if there is a failure or fault condition, there will be frozen pipes, process fluid degradations and other costly problems. Also, the self-regulating heating cables require a lot of energy to operate and there is no provision for energy conservation in freezing conditions.
In order to provide a system for monitoring and controlling the heating cable or heat tracing circuit, a third conductor was added to the heating cable and a monitoring system, for example, the MoniTrace 1000 by Raychem Corporation uses the third conductor in addition to the two bus conductors to monitor and control the heating cable and it uses a thermostat for energy conservation.
Among the disadvantages of this type of system, is that the three conductor cable is more expensive than the two conductor cable.